1. Field of the Invention
The present invention relates to a focusing control apparatus and, more particularly, to a focusing control apparatus which performs a control operation to attain an optimum light converging position of read light that is irradiated onto a predetermined recording surface of a recording medium, with regard to the recording surface.
2. Description of Related Art
Nowadays, an information recording medium of high recording density and large capacity called "digital video disc or digital versatile disc (abbreviation: DVD)" and systems using the medium are used widely. The disc includes a type having information recording surfaces formed on upper and lower two layers sandwiching an interlayer part (spacer or space region). When data is read by an optical pickup from one of the disc surface sides of the disc of such a type, a focal point (focal position or optimum light converging position) of read light has to be formed on the information recording surface of a desired one of the layers.
Usually, a focus jumping operation for moving a focal point of read light from an information recording surface of one of the layers to an information recording surface of the other layer is executed based on the detection of a zero-crossing point of a focusing error signal that is generated based on an output signal of a pickup.
Specifically, the pickup has a focusing actuator for displacing the focal point of read light in the direction perpendicular to the disc surface by displacing an objective lens which is, for example, a read light radiating optical system and which determines its focal point in the optical axial direction. An acceleration signal as a focus jump activation signal for moving the focal point of the read light to a target recording surface is supplied to the focusing actuator at the initial stage of the focus jumping operation. Based on the timing of a zero-crossing point which is sequentially detected from the focusing error signal obtained during the displacement of the focusing actuator in response to the acceleration signal, a series of operations for finishing the supply of the acceleration signal, supplying a deceleration signal to stop the displacement of the focusing actuator in response to the acceleration signal to the focusing actuator and, further, finishing the supply of the deceleration signal and restarting a focusing servo to the target recording surface is executed.
FIG. 1 shows a relationship between a position in a disc internal layer of a focal point P of read light which is determined by an objective lens 100 and a level FE of a focusing error signal obtained when the focal point P is moved in an optical axial direction. It will be understood that the focusing error signal basically shows the zero level (zero-crossing points ZC00 and ZC10) in a state where the focal point P is on the recording surface and draws an S-shaped curve around a zero level as the center. A period from the minimum value to the maximum value of one S-shaped curve substantially corresponds to a control range of a focusing servo loop which is formed. Since the supply control of the acceleration signal and deceleration signal is executed during the focal point movement between one recording surface and another recording surface, it is performed on the basis of detection timings of zero-crossing points ZC00, ZC01, ZC1-1, and ZC10 of the focusing error signal as shown in the diagram.
The focusing error signal obtained during the movement of the focal point between the recording surfaces is, however, influenced by defects on the surface or the recording surface of the disc or on the surface of the other internal layer, an improper formation mark which is equivalent to the defects (hereinafter, referred to as defects, generally), or a crosstalk. Specifically, when the defect position is scanned by the read light or the crosstalk occurs, the level of the generated focusing error signal remarkably drops or sharply increases. There is, therefore, a possibility that the focusing error signal shows what is called a pseudo zero-crossing point different from a desired zero-crossing point by the S-shaped curve shown in FIG. 1. In this case, the supply control of the acceleration signal and deceleration signal cannot be correctly executed, the focusing actuator is consequently erroneously operated, and a focus jumping operation fails.
Hitherto, since the focus jumping operation is executed even when a drop-out occurs in a read signal and even when a vibration occurs in the disc or the player, the jumping operation often fails. This type of failure of the jumping operation occurs irrespective of the above-mentioned defects and the detection of zero-crossing.